Onmouseover removed from span tags

I am having problems with my attributes being removed from span tags.
I have been reading about this issue on the forum but I don’t seem to see any real solutions.
To get the span tags to remain I added to functions.php

function myextensionTinyMCE($init) {
    $ext = 'span[id|name|class|style]';
    if ( isset( $init['extended_valid_elements'] ) ) {
        $init['extended_valid_elements'] .= ',' . $ext;
    } else {
        $init['extended_valid_elements'] = $ext;
    }
    return $init;
}
add_filter('tiny_mce_before_init', 'myextensionTinyMCE' );

So now the span tags themselves remain however I have an unordered list that looks like this:

<ul>
<li><span onmouseover="show_image('one.jpg')">Phil Butendorp</span></li>
<li><span onmouseover="show_image('two.jpg')">Ted Some</span></li>
</ul>

but if you switch to the visual tab the attribute is removed

<ul>
<li><span>Phil Butendorp</span></li>
<li><span>Ted Some</span></li>
</ul>

Does anyone know how to get this to work?

thank you